H2—O2 fuel cells, particularly Proton Exchange Membrane (PEM) fuel cells, use hydrogen (H2) as a fuel and oxygen (typically from air) as an oxidant to produce electricity. The hydrogen used in the fuel cell can be derived from the reformation of a hydrocarbon fuel (e.g., methanol or gasoline) in a primary reactor. For example, in a steam reforming process, a hydrocarbon fuel (such as methanol) and water (as steam) are ideally reacted in a catalytic reactor (commonly referred to as a “steam reformer”) to generate a reformate gas comprising primarily hydrogen and carbon monoxide. An exemplary steam reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh.
For another example, in an autothermal reformation process, a hydrocarbon fuel (such as gasoline), air and steam are ideally reacted in a combined partial oxidation and steam reforming reactor (commonly referred to as an autothermal reformer or ATR) to generate a reformate gas containing hydrogen and carbon monoxide. An exemplary autothermal reformer is described in U.S. Pat. No. 6,521,204 to Borup et al. The reformate gas exiting the reformer, however, contains undesirably high concentrations of carbon monoxide, most of which must be removed to avoid poisoning the anode catalyst of the fuel cell. In this regard, the relatively high level of carbon monoxide (i.e., about 3-10 mole %) contained in the H2-containing reformate exiting the reformer must be reduced to relatively low concentrations (e.g., less than 200 ppm, and typically less than about 20 ppm) to avoid poisoning the anode catalyst. Thus, a fuel processing system used in connection with a fuel cell typically employ secondary or CO cleanup reactors to achieve a stack-grade reformate stream.
As noted above, conventional steam reforming typically comprises introducing a mixed stream of a hydrocarbon fuel and steam into a catalyst bed that is maintained at operating temperature by an external heating source. Autothermal reforming, on the other hand, typically comprises introducing a mixed stream of air, steam and a hydrocarbon fuel into a reactor containing a catalyst bed with the necessary heat being supplied by an exothermic reaction between oxygen and the fuel. Ideally, a reformer will convert a hydrocarbon fuel into a H2-containing reformate while maximizing the heat utilization and simultaneously minimizing the conversion temperature, methane production, and carbon formation. Further, for vehicular applications, the reformer will ideally achieve this conversion in a compact reactor system that can be closely integrated with the fuel cell system that will be consuming the hydrogen produced.
Both conventional steam reforming and autothermal reforming have certain performance limitations inherent in their heat utilization schemes. For example, while steam reformers increase efficiency by utilizing heat from hot waste streams in the endothermic catalysis, the design of autothermal reformers precludes recovering much of the waste heat and reduces their efficiency. However, while heat transfer limitations slow the response of steam reformers to transient operation demands, autothermal reformers can more easily accommodate varying system demands. Further, steam reformers are typically larger than autothermal reformers, making autothermal more suitable for portable and vehicular applications.
It is known that the carbon monoxide, CO, level of the reformate exiting a reformer can be reduced by utilizing a so-called “water gas shift” (WGS) reaction wherein water (typically in the form of steam) is combined with the reformate exiting the reformer, in the presence of a suitable catalyst. Some of the carbon monoxide (e.g., as much as about 0.5 mole % or more) will survive the shift reaction so that the shift reactor effluent will comprise hydrogen, carbon dioxide, water, carbon monoxide, and nitrogen.
As a result, the shift reaction alone is typically not adequate to reduce the CO content of the reformate to levels sufficiently low (e.g., below 200 ppm and preferably below 20 ppm) to prevent poisoning the anode catalyst. It remains necessary, therefore, to remove additional carbon monoxide from the reformate stream exiting the shift reactor before supplying it to the fuel cell. One technique known for further reducing the CO content of reformate exiting the shift reactor utilizes a so-called “PrOx” (i.e., Preferential Oxidation) reaction conducted in a suitable PrOx reactor under conditions which promote the preferential oxidation of the CO without simultaneously consuming/oxidizing substantial quantities of the H2 fuel or triggering the so-called “reverse water gas shift” (RWGS) reaction. About four times the stoichiometric amount of O2 is used to react with the CO present in the reformate to ensure sufficient oxidation of the CO without consuming undue quantities of the H2.
Primary reactors for gasoline or other hydrocarbons typically operate at high temperatures (i.e., about 600-800° C.), with water gas shift reactors generally operating at lower temperatures of about 250-450° C., and the PrOx reactors operating at even lower temperatures of about 100-200° C. Thus, it is necessary that the reformer, the water gas shift (WGS) reactor, and the PrOx reactor are each heated to temperatures within their operating ranges for the fuel processor in a start-up mode prior to operating as designed. During the start-up of a conventional fuel processor, however, the heating of various components is typically sequentially staged. This sequential approach to heating can lead to undesirable lag time for bringing the system on line. Alternately, external electrical heat sources (i.e., resistance heaters) may be employed to bring the components to proper operating temperatures more quickly, but this approach requires an external source of electricity such as a battery.
Accordingly, there exists a need in the relevant art to provide a fuel processor that can operate in a regime between a steam reformer and an autothermal reformer. Furthermore, there exists a need in the relevant art to provide a fuel processor capable of heating the various components while minimizing the consumption of electrical energy during startup and the reliance on catalytic reactions. And further, there exists a need for a fuel processor that can be used successfully with a range of hydrocarbon fuels and in a variety of transient operating conditions.